In the preparation of pulp for use in the manufacture of paper, a common process includes the digesting of wood chips in pulping liquor to break down the pulp into individual fibers and bunches of fibers by dissolving the substances, such as lignins, which bind the fibers together. The spent pulping liquor will therefore contain such dissolved substances and spent chemicals.
The preparation of pulp from wood chips may require two or more separate pulp washing operations. For example, the ground wood or wood chips are first processed with chemicals under pressure and temperature, usually by either the kraft alkaline process or by the sulfite acid process. In either process, digestion dissolves the lignins thereby freeing the fibers and placing the lignin components into solution. In both processes the resulting liquor is dark in color, and the residual liquor which does not drain from the pulp and the remaining contaminants must be washed from the pulp. Further, it is desirable to recover spent liquor at as high a concentration as practical to minimize the cost of the subsequent recovery of chemicals.
Brown pulp which has been so washed retains a definite brown color and the pulp which remains is usually too highly colored for making white paper. Also, if any lignin is present, paper made from such pulp may not have a high degree of permanence and will yellow in time. Therefore, it is common and conventional to apply a bleaching process to the pulp, not only to improve whiteness, but to improve permanence of the whiteness.
The bleaching commonly is performed in a chlorination stage by applying a water in which chlorine gas has been dissolved. Other bleaching processes may be used, such as a sodium hydrosulphite process, as is well known in the art. Bleaching may not be accomplished in a single stage and may be performed in two or more stages, each followed by washing. After bleach treatments, the pulp is subjected to a washing action to remove the water which contains the spent bleaching agents and dissolved lignin.
A particularly effective and useful pulp washer is manufactured and sold by The Black Clawson Company, the assignee of this application, under the trade name CHEMI-WASHER, in accordance with the teachings of Ericsson, U.S. Pat. No. 4,154,644 issued May 15, 1979, the disclosure of which is incorporated herein by reference. The Ericsson apparatus is an efficient flat bed countercurrent washer. It has a construction similar to that of a Fourdrinier paper machine, in that it incorporates an endless foraminous belt or wire, and a headbox which delivers the pulp suspension and liquor to one end of the wire. The pulp is subjected to successive washing zones or stages as it is carried by the wire through the machine.
In the operation of the Ericsson washer, a suspension of the pulp is diluted to a sufficiently low inlet consistency, such as 1.5% to 3.0%, and is deposited at the upstream end of the wire run where a mat of fiber is formed in a forming stage as the liquid drains through the wire. After the mat is formed on the wire it is partially dewatered to a displacement consistency of about 10% to 12% in the forming zone.
The machine downstream from the headbox and the forming zone is divided into a series of washing zones or stages to which a washing liquid is applied from above for drainage through the mat. The freshest or cleanest washing liquid is applied to the zone nearest the off-running end of the wire and the liquid drained through the mat at that zone is collected from the suction boxes and delivered to the immediately preceding washing zone. This is repeated from zone to zone, so that the cleanest pulp is treated with the cleanest water, and the dirtiest pulp is treated with the dirtiest water.
The requirement for an oxygen treatment or a bleaching process followed by a further washing, frequently results in a duplication of washing equipment or the addition of washing apparatus dedicated to washing the bleached pulp. Typically, such separate washing equipment may be in the form of a decker. The duplication of pulp washing equipment not only requires a duplication of apparatus, but in many instances, a duplication of the use of washing water, energy and floor space.